Regeneration method for a plating solution

ABSTRACT

The invention relates to a method of depositing a layer of metal and to a method of regenerating a solution containing metal ions in a high oxidation state. To regenerate tin ions consumed from a tin plating solution by metal deposition, it has been known in the art to carry the plating solution over metallic tin to cause tin (II) ions to form. However, the amount of tin contained in thus regenerated baths slowly and continuously increases. The solution to this problem is to utilize an electrolytic regeneration cell that is provided with at least one auxiliary cathode and with at least one auxiliary anode. Tin serving for regeneration is electrolytically deposited from the solution onto the at least one auxiliary cathode in the electrolytic regeneration cell. The solution is carried over the tin serving for regeneration in order to reduce formed tin (IV) ions to tin (II) ions.

[0001] The invention relates to a method of depositing a layer of metal,more specifically a layer containing tin, above all for fabricatingprinted circuit boards and other electrical circuit carriers, and to amethod of regenerating a solution containing metal ions in a highoxidation state, more specifically Sn(IV) ions. The plating method ismainly intended for utilization in the production of solderable layersand etch-resist layers as well as in the deposition by cementation oflayers of tin onto conductive patterns made of copper more specificallyon the inner layers of printed circuit boards in order to bond saidinner layers together.

[0002] For fabricating printed circuit boards, layers of tin and tinalloys, more specifically tin-lead coatings are deposited onto thecopper surfaces to serve diverse purposes.

[0003] On the one side, tin-lead alloy coatings serve as solder pads onthe surface of the printed circuit board at the places at whichelectronic component parts are to be soldered. In this case, such layersare deposited locally in those regions in which leads or otherconnecting elements of the component parts are to be electricallyconnected to the copper surface. After the solder regions are formed onthe copper surfaces, the components are mounted on the solder pads wherethey are bonded. Next, the solder is remelted in an oven to allow theelectrical interconnections to form.

[0004] Layers of tin may also be used as etch-resist layers, e.g., toform metal patterns on the surfaces of the printed circuit boards. Forthis purpose, a negative image of the conductive pattern is at firstformed on the copper surfaces by means of a photo-patternable resist.Then, the layers of tin or of tin-lead alloy are deposited in the canalsof the resist layer. After the resist is removed, bare copper may beremoved by etching so that it is only the circuit traces and all theother metal patterns on the surfaces of the printed circuit board thatremain below the layer of tin or tin-lead.

[0005] Furthermore, tin layers are also utilized as intermediate layersbetween the copper surfaces of the inner layers of multilayered circuitboards and the areas of the dielectric (usually glass fiber reinforcedlayers of resin). For to provide tight bonding of the copper areas withthe dielectric, it is necessary to roughen the copper surfaces prior topressing in order to achieve sufficient bonding strength between copperand resin. To accomplish this, the surfaces have heretofore beensuperficially oxidized by a so called black oxide treatment. However,the thereby formed oxide layer is not sufficiently resistant to acids sothat the inner layers, which have been cut in the process of drillingthe PCB material, are delaminated from the resin of the PCB material,forming delaminations. This problem is avoided when tin layers are usedinstead of the black oxide layers. For production, the tin layers aredirectly deposited by cementation onto the copper surfaces of thecircuit traces. In post-treatment, if necessary, further bondingcompounds are applied to the tin layers (e.g., a mixture of anureidosilane and a disilane cross-linking agent (EP 0 545 216 A2))before the inner layers are pressed together by action of heat andpressure.

[0006] Whereas, in the second application mentioned, the layers of tinor tin-lead alloy, respectively, can be electrolytically deposited as noelectrically isolated metal regions have to be tin-plated, in the firstand in the last mentioned case tin cannot be deposited by means of anelectrolytic method since the copper areas to be metal plated usuallyare electrically mutually isolated so that it is hardly possible toestablish an electric contact. For this reason, so called cementationbaths are at hand for tin-plating.

[0007] A plating bath of this type is described in U.S. Pat. No.4,715,894. In addition to a Sn(II) compound, this bath also contains athiourea compound and an urea compound. According to EP 0 545 216 A2,thiourea, urea and the derivatives thereof may also be used asalternatives. Furthermore, the solution in accordance with U.S. Pat. No.4,715,894 also contains a complexing agent, a reducing agent and anacid. Accordingly, the Sn(II) compound used is SnSO₄ for example.According to EP 0 545 216 A2, the bath contains Sn(II) compounds ofinorganic (mineral) acids, for example compounds of acids containingsulfur, phosphorus and halogen, or of organic acids such as Sn(II)formiate and Sn(II) acetate for example. According to EP 0 545 216 A2,the Sn(II) salts of the acids containing sulfur are preferred i.e., thesalts of sulfuric acid and of sulfamic acid. Furthermore, the bath mayalso contain alkali metal stannates such as sodium stannate or potassiumstannate. Moreover, the thiourea and the urea compounds are, in thesimplest case, the unsubstituted derivatives of thiourea and urea,respectively. According to EP 0 545 216 A2, Cu(I) ions complexed withthiourea are to form onto the copper surfaces when tin is deposited.Concurrently, metallic tin is deposited by reduction of Sn(II) ions. Inthis reaction, copper is dissolved, a tin coating being simultaneouslyformed on the copper surfaces.

[0008] EP 0 545 216 A2 reports that the Cu(I) thiourea complex enrichesin the solution. Sn(IV) ions also enrich in the solution throughoxidation of Sn(II) ions as oxygen from the air is carried into thesolution. However, the concentrations of the Cu(I) thiourea complex andof the Sn(IV) ions do not exceed stationary concentration values whenthe printed circuit boards are merely immersed into the solution fortreatment, since the bath solution is permanently drained away by theboards and diluted with water that has been carried over. If however,the bath fluid is sprayed onto the copper surfaces by way of spraynozzles, the rate of substance turnover, related to the volume of thebath, is considerably higher. Under these conditions, the concentrationof the Cu(I) thiourea complex increases to such an extent that the limitof its solubility is reached and the complex precipitates as a deposit.The deposit clogs the nozzles and causes problems in the movablemechanical parts of the plant. In the plating bath, Sn(IV) compounds arealso increasingly formed by oxidation of the Sn(II) ions through oxygenfrom the air as air is carried to a greater extent into the bathsolution by spraying the latter onto the printed circuit boards.

[0009] To mitigate these problems, the following provisions have beendescribed in the publication mentioned: to reduce the concentration ofthe Cu(I) thiourea complex, part of the solution of the plating bath istaken from the treatment container to another tank where it is left tocool down so that a large part of the complex precipitates and can thusbe separated. The solution, which is now largely freed from the complex,may then be returned to the treatment container. To further lower theconcentration of the Sn(IV) ions in the plating solution, there isprovided a reservoir for the plating solution that contains metallictin. The solution contained in said reservoir is sprayed onto the coppersurfaces, the Sn(II) ions being reduced according to the reactionequation (1) set forth below, and metallic copper simultaneouslyoxidizing to form Cu(I) ions according to reaction equation (2) which isalso set forth below. A complex with thiourea or with the derivativesthereof, respectively, is formed thereby. Simultaneously, through theoxygen carried into the solution, part of the Sn(II) ions oxidizes toform Sn(IV) ions according to the reaction equation (3) set forth below.The sprayed solution is next returned to the reservoir. There, theSn(IV) ions react with the metallic tin to form the double quantity ofSn(II) ions according to the reaction equation (4) set forth below.

[0010] The method of regenerating tin-plating cementation bathsdescribed in EP 0 545 216 A2 proved however to cause the concentrationof tin contained in the solution to rise continuously. Therefore, theconcentration of Sn(II) ions in the solution must be subjected topermanent analytic control. This is often not easily possible undermanufacturing conditions and often readily causes the concentration tovary greatly. As a result thereof, the deposition of tin can becomeuncontrollable. This is not acceptable. One approach to overcoming thisproblem could involve automated monitoring of the concentration ofSn(II) ions and permitting or obviating contact between the platingsolution and metallic tin in the reservoir when a predetermined range ofreference values is exceeded or not reached, respectively. This is verycomplicated though and requires quite complicated devices.

[0011] It is therefore an object of the present invention to overcomethe problems mentioned and to find means permitting tin plating ofcopper surfaces by cementation without variations of the Sn(II) ionscontent affecting the deposition of tin. It is aimed at making thispossible without the use of complicated devices.

[0012] A solution to this object is the plating method of claim 1 andthe regeneration method of claim 14. Preferred embodiments of theinvention are indicated in the subordinate claims.

[0013] The plating method in accordance with the invention serves toproduce layers of metal, more specifically layers containing tin andpreferably layers of pure tin. The method can also be utilized fordepositing layers consisting of a tin alloy. It involves the followingmethod steps:

[0014] a. Providing a metal plating bath, more specifically a tinplating bath; containing metal ions in a low oxidation state, morespecifically Sn(II) ions,

[0015] b. Depositing a metal layer from the metal plating bath onto awork piece;

[0016] c. Providing an electrolytic regeneration cell comprised of atleast one auxiliary cathode and of at least one auxiliary anode;

[0017] d. Electrolytically depositing, in the electrolytic regenerationcell, metal serving for regeneration, more specifically metallic tin,from the metal plating bath onto the at least one auxiliary cathode;

[0018] e. Bringing the metal plating bath into contact with the metalserving for regeneration in an effort to reduce metal ions in a highoxidation state contained in the metal plating bath, more specificallySn(IV) ions, to metal ions in a low oxidation state, more specificallySn(II) ions.

[0019] The regeneration method of the invention serves to regeneratesolutions containing metal ions in a high oxidation state, morespecifically Sn(IV) ions in order to reduce the metal ions in the highoxidation state to metal ions in a low oxidation state, morespecifically to Sn(II) ions. This comprises the following method steps:

[0020] a. Providing an electrolytic regeneration cell comprised of atleast one auxiliary cathode and of at least one auxiliary anode;

[0021] b. Electrolytically depositing, in the electrolytic regenerationcell, metal serving for regeneration, more specifically metallic tin,from the solution onto the at least one auxiliary cathode;

[0022] c. Bringing the solution into contact with the metal serving forregeneration in an effort to reduce metal ions in the high oxidationstate, more specifically Sn(IV) ions, to metal ions in the low oxidationstate, more specifically Sn(II) ions.

[0023] When hereinafter layers containing tin, a tin plating bath or atin plating solution, metallic tin, Sn(II) ions, Sn(IV) ions and a tinelectrode or an electrode containing tin, respectively, are referred to,this should also apply generally and in lieu of to metal layers, a metalplating bath, metal, metal ions in a low oxidation state, metal ions ina high oxidation state, a metal electrode or an electrode containingmetal, respectively.

[0024] The methods of the invention may more specifically be utilizedfor electroless deposition of tin or tin alloys utilizing a reductionagent, for the electrolytic deposition of tin and tin alloys and for thedeposition by cementation of tin or tin alloys.

[0025] By a method of deposition by cementation a method is meant bywhich the metal to be deposited receives from the substrate metal theelectrons needed for the reduction to the oxidation state zero, saidsubstrate metal concurrently oxidizing and being preferably dissolvedthereby.

[0026] The method of the invention more specifically serves to coatcopper surfaces on printed circuit boards or other circuit carriers withtin containing layers.

[0027] In the method described in EP 0 545 216 A2, metallic tin is addedto the plating solution contained in the reservoir in order to convertSn(IV) ions to Sn(II) ions. By contrast, with the method of theinvention, the metallic tin utilized for regeneration is produced byelectroplating it from the very tin plating bath. The method of theinvention thus permits to avoid variations in the concentration ofSn(II) ions contained in the plating bath. This can be explained asfollows:

[0028] When tin is deposited from an electroless, cementation orelectrolytic tin bath, the following reaction takes place:

Sn²⁺+2e ⁻--------------->2Sn  (1)

[0029] In electrolytic deposition, the electrons originate from anexternal source of electric current and are delivered to the Sn(II) ionsvia the cathode. In the case of electroless tin plating, the electronsneeded for depositing the metal are provided by a reduction agent. Indeposition by cementation, the electrons originate from the dissolvingbase metal, in the present case copper, onto which tin is deposited:

2Cu---------------->2Cu⁺+2e ⁻  (2)

[0030] In an interfering side reaction, Sn(II) ions oxidize in thesebaths, through the oxygen from the air, to form Sn(IV) ions:

Sn²⁺+½O₂+H₂O--------->Sn⁴⁺+2OH⁻  (3)

[0031] The Sn(IV) ions formed tend to precipitate tinstone (SnO₂). Theproblems related therewith are, inter alia that spray nozzles fordelivering the plating solution to the copper surfaces may clog and thatthe function of movable parts in the processing plant may be impaired orthe parts may even be damaged by precipitating solid matter.Furthermore, the Sn(IV) ions also have the disadvantageous property thatthe layer of tin, freshly deposited according to the reaction equation(1), is attacked by the Sn(IV) ions according to the reaction equation(4) set forth herein below, so that it may be dissolved again, at leastpartially.

[0032] In contacting the plating solution with metallic tin, Sn(IV) ionscontained in the solution are reduced to Sn(II) ions according to theequation set forth below for this reaction, metallic tin being dissolvedin the process (comproportionation):

Sn⁴⁺+Sn---------->2Sn²⁺  (4)

[0033] This means that for each Sn(IV) ion formed, two Sn(II) ions areformed. As a result thereof, the concentration of tin contained in theplating solution increases gradually when the regeneration methodaccording to EP 0 545 216 A2 is applied.

[0034] By contrast, in carrying out the method of the invention, themetallic tin used for reducing the Sn(IV) ions originates throughelectrolytic deposition from the very tin plating solution. As a resultthereof, the tin balance of the bath is not disturbed by theregeneration according to equation (4). As the metallic tin used forregeneration is also formed from Sn(II) ions according to equation (1),and hence the concentration of the Sn(II) ions being lowered at first byelectrolytic deposition, the Sn(II) ions consumed both through thisreaction (1) and through the side reaction (3) are produced again by theregeneration reaction (4). The Sn(II) ions content therefore remainsconstant.

[0035] The method of the invention therefore permits to avoid thedetrimental consequences resulting from the formation of Sn(IV) ions andto concurrently regenerate the Sn(II) ions from the Sn(IV) ions withoutcomplicated devices and analytic expenditure.

[0036] The plating solution substantially contains at least one Sn(II)compound, at least one compound from the group comprising thiourea, ureaand the derivatives thereof as well as at least one acid. If a tin alloyis deposited, the solution additionally contains at least one salt ofthe metal to be deposited additionally, e.g., one or more nickel, lead,mercury and/or gold salts. Furthermore, the tin plating solution mayalso contain complexing agents, reducing agents as well as othercomponent parts, like stabilizing agents for controlling deposition andfor making sure that the plating solution be stable to decomposition, aswell as surface-active agents. Usually, the solution is aqueous, i.e.,the solvent contained in the solution consists of at least 50 percent byvolume of water. It may also contain organic solvents like for examplealcohols and ether esters.

[0037] The Sn(II) compound is preferably a Sn(II) salt of an inorganic(mineral) acid, e.g., of an acid containing sulfur, phosphorus and/orhalogen; hydrogen halides however should be avoided because of theircorrosive effect and their tendency to incorporate tin halides into thedeposited tin. Furthermore, the Sn(II) compound may also be the Sn(II)salt of an organic acid, e.g., of Sn(II) formiate, Sn(II) acetate andthe homologues thereof and the salt of an aromatic acid, morespecifically of Sn(II) benzoate. The preferred salts are the Sn(II)salts of the acids containing sulfur, i.e., the salts of the sulfuricacid and of the sulfamic acid (SnSO₄ and Sn(OSO₂NH₂)₂). The solution mayfurthermore contain alkali metal stannates such as sodium stannate orpotassium stannate.

[0038] If a tin alloy is deposited, the tin plating solutionadditionally contains at least one compound of the other alloyingmetals, for example a nickel, lead, mercury and/or gold salt; the anionsof these salts can be the same as those utilized for the tin salts.

[0039] With respect to the Sn(II) compounds and to the compounds ofother alloying metals, reference is made to U.S. Pat. No. 4,715,894. Thecompounds disclosed therein are incorporated herein by reference as adisclosure.

[0040] The acid contained in the tin plating solution preferably is amineral acid but may also be an organic acid, the anion of the acidbeing generally identical with that of the tin salt and, if necessary,with that of the salts of the other alloying metals.

[0041] The compounds of thiourea and urea used are more specifically theunsubstituted derivatives (thiourea, urea), the solution generallycontaining only thiourea and/or the derivatives thereof. U.S. Pat. No.4,715,894 indicates suitable derivatives of thiourea and of urea. Thederivatives disclosed therein are incorporated herein by reference as adisclosure.

[0042] The tin plating solution can also contain complexing agents,those indicated in Kirk-Othmer, Encyclopedia of Chemical Technology,3^(rd) Edition, Volume 5, pages 339-368 being particularly suited. Thecomplexing agents disclosed therein are incorporated herein as adisclosure. More specifically, amino carboxylic acids and hydroxycarboxylic acids may be used. U.S. Pat. No. 4,715,894 discloses certainexamples of suitable compounds. The complexing agents disclosed thereinare incorporated herein by reference as a disclosure.

[0043] The solution may also contain reducing agents, aldehydes, e.g.,formaldehyde and acetaldehyde being more specifically utilized. Furtherreducing agents are indicated in U.S. Pat. No. 4,715,894. The reducingagents disclosed therein are incorporated herein by reference as adisclosure.

[0044] Anionic, cationic and amphoteric surface-active agents may beused alike. It only matters that the surface-active agents are suited toreduce the surface tension of the plating solution sufficiently.

[0045] The metallic tin used for regeneration may be deposited onto aninert auxiliary cathode. By inert cathode, a separate electrode is meantwhich consists of a material that resists dissolution in the tin platingsolution when the electrode is subjected to anodic polarization. Morespecifically, the auxiliary cathode can be made of platinized titanium.

[0046] The auxiliary cathode can be configured as a plate, a tube,expanded metal or as a formed body like for example a plate providedwith ribs. The auxiliary cathode may also be shaped in smaller pieces,e.g., in the shape of spheres having for example a diameter of some fewmillimeters to some few centimeters. In the latter case, these piecesmay be accommodated in a separate container for example, the platingsolution flowing through said container. For this purpose, the piecesmay for example be placed on a perforated bottom plate accommodated in atower, the plating solution entering through said bottom plate andflowing through said tower. Configuring the auxiliary cathode in theform of smaller pieces permits to considerably increase the conversionrate of the Sn(IV) ions to Sn(II) ions.

[0047] If an inert auxiliary cathode is made use of, the maximumquantity of tin that can be dissolved again in the regeneration reactionaccording to reaction equation (4) is that amount that had beenpreviously deposited from the bath. As a result thereof, the bath can beregenerated continuously without complicated analytical bath monitoringand, by contrast to the method according to EP 0 545 216 A2, theconcentration of tin in the bath does not rise.

[0048] If, for depositing tin onto platinized titanium for example, thecathodic current density set for the auxiliary cathode is sufficientlyhigh (e.g., 8 A/dm²), a tin coating in the form of flat scale crystalsis obtained. This crystal shape has a very large surface which is wellsuited for the regeneration reaction according to equation (4) since itprovides a very large surface referred to the weight of tin. As a resultthereof, a large surface of deposited tin can be provided in apredetermined volume of plating solution. A similar scale deposition isalso observed when a high current density is produced on the auxiliarycathode when said auxiliary cathode is made of copper or of a copperalloy, for example with silver. The advantage of copper over inertmaterials, for example platinized titanium, is that copper is lessexpensive. The durable life of this material in a chemical tin platingsolution is limited though.

[0049] The auxiliary cathode is in electric contact with the platingsolution. An auxiliary anode, which is in direct electric contact withthe plating solution or which is in electric contact with the platingsolution via another solution, is also provided. By application ofvoltage between the auxiliary cathode and the auxiliary anode, a flow ofcurrent can be generated between these two electrodes, the auxiliarycathode being polarized cathodically and the auxiliary anode beingpolarized anodically when tin is to be deposited onto the auxiliarycathode. If tin deposited onto the auxiliary cathode is directlyutilized to regenerate the tin plating solution, the auxiliary cathodeis not to be polarized cathodically during the actual regenerationprocess in order to allow the tin to dissolve from the auxiliarycathode. Therefore, with this method, the auxiliary cathode is onlypolarized cathodic intermittently each time tin is to be deposited ontothe auxiliary cathode. As soon as enough tin has been deposited onto theauxiliary cathode, the electrical connection between the auxiliarycathode and the auxiliary anode is interrupted in order to halt thedeposition process. Then, the dissolution reaction according to equation(4) of this reaction takes place under these conditions, the platingsolution having to be contacted with the auxiliary cathode. As soon asbut a small amount of tin or no tin at all is left at the auxiliarycathode, tin can again be deposited onto said electrode.

[0050] For the regeneration reaction, metallic tin formed on theauxiliary cathode may either be used directly in contacting the platingsolution with the auxiliary cathode coated with metallic tin or beremoved mechanically from said electrode and be contacted with the tinplating solution after removal thereof. To mechanically remove the tindeposited onto the auxiliary cathode, the auxiliary cathode ispreferably taken out of the plant and the scales of metal that havegrown thereon are stripped off. The removed tin may then be placed intothe container for treating the printed circuit boards or into areservoir that contains the tin plating solution. In the treatmentcontainer or in the reservoir, the tin dissolves to form Sn(II) ions,Sn(IV) ions being consumed in the process. As soon as the whole quantityor at least almost the whole quantity of tin placed in the container orin the reservoir has dissolved, further tin that has deposited onto theauxiliary cathode may be added.

[0051] The rate at which tin from the very auxiliary cathode or metallictin removed from the auxiliary cathode and placed into the treatmentcontainer or into a reservoir dissolves in the plating solution dependson a plurality of parameters: the dissolution rate of tin depends interalia on the composition and on the temperature of the plating bath, onthe morphology of electrolytically deposited tin, on the geometricalsurface of the auxiliary cathode and on the flow conditions in immediateproximity to the dissolving tin. The rate may thus be optimized. Amaximum dissolution rate is permanently aimed at since under theseconditions Sn(IV) ions are actually quantitatively reduced to Sn(II)ions. This makes it possible to minimize the concentration of Sn(IV)ions contained in the plating solution. The dissolution rate is thehigher the higher the concentration of acid in the tin plating solution,the higher the temperature of the bath, the larger the surface of tindeposited onto the auxiliary cathode, referred to the weight of the tin,the larger the geometrical surface of the auxiliary cathode and thehigher the convection of the plating solution in immediate proximity tothe dissolving tin.

[0052] To optimize the method of the invention, the space surroundingthe auxiliary anode (anode space) in the electrolytic regeneration cellcan be separated from the space surrounding the auxiliary cathode(cathode space) by a membrane. The membrane is preferably configured insuch a manner that cations (Sn(II) ions and Sn(IV) ions) cannot passthrough. Therefore, the membrane may more specifically be an anionexchange membrane or a monoselective ion exchange membrane. In aparticularly preferred embodiment of the method in accordance with theinvention, there is an acid in the anode space. The acid contained inthe plating solution in the cathode space and the acid contained in theanode space may be identical. However, a very good regeneration resultis also obtained when the acid contained in the tin plating solutiondiffers from the acid in the solution contained in the anode space. Forexample a tin plating solution containing methane sulfonic acid and asulfuric acid solution contained in the cathode space yield goodresults. There is transfer of fluid between the cathode space and theregion in which layers containing tin are deposited onto the printedcircuit boards.

[0053] These further improvements of the method in accordance with theinvention permit to prevent the tin plating bath from directlycontacting the auxiliary anode. Sn(IV) ions are thus prevented fromforming at the auxiliary anode, which would otherwise lower theefficiency of regeneration. The auxiliary anode may for example beimmersed into an anode space that is separated from the cathode spacesurrounding the auxiliary cathode by an anion exchange membrane. Theplating solution in the cathode space, which more specifically containsSnSO₄ and H₂SO₄ for example, cannot get near the auxiliary anode sincethe membrane prevents Sn(II) ions from passing through. A solution ofthe acid which is also contained in the cathode space is preferably alsofilled into the anode space. In the present example, the acid would beH₂SO₄. When the current flows between the two spaces, electroneutralityis guaranteed by the transfer of sulfate anions and by the correspondingelectrode reactions, i.e., by the tin plating reaction at the auxiliarycathode according to equation (1) of this reaction and by an oxidationreaction at the auxiliary anode, in which oxygen is formed from wateraccording reaction equation (5):

2H₂O--------------->2H⁺+2e ⁻+O₂  (5)

[0054] As the Sn(II) ions are prevented from contacting the auxiliaryanode, oxidation of Sn(II) ions according to the following equation:

Sn²⁺------------------->Sn⁴⁺+2e ⁻  (6)

[0055] cannot take place.

[0056] Alternatively, the auxiliary anode can also contact the tinplating solution directly. In order to also prevent in this caseoxidation of the Sn(II) ions according to reaction equation (6), theconcentration overvoltage must be high enough for this reaction. Thismay be realized by an appropriate geometrical arrangement of theauxiliary anode relative to the auxiliary cathode for example: adepletion of the Sn(II) ions in the solution in the immediate proximityto the auxiliary cathode, which may lead to the concentrationovervoltage, may also be achieved in that the anode space isaccommodated in a container which is separated from the cathode space,both spaces communicating through a pipe whose diameter is relativelysmall.

[0057] Concentration overvoltage in the above mentioned sense may alsobe achieved in considerably increasing the current density at theauxiliary anode so that Sn(II) ions are virtually no longer available inthe immediate proximity of the auxiliary anode. Under these conditions,Sn(II) ions do not oxidize to form Sn(IV) ions, but water oxidizes toform oxygen. The current density at the auxiliary anode may for examplebe increased by reducing the surface of the auxiliary anode relative tothe surface of the auxiliary cathode.

[0058] In another embodiment of the invention, at least one electrodecontaining the tin to be deposited, i.e., an electrode of metallic tinfor example, can be contacted with the tin plating bath. This tinelectrode is polarized anodically relative to another electrode so thatthe tin electrode dissolves at least partially. Such a soluble tinelectrode may for example consist of poured balls which are located in asuitable container, e.g., in a titanium basket.

[0059] In this case, the tin electrode is at least intermittentlypolarized anodic relative to the other electrode so that metallic tindissolves to form Sn(II) ions.

[0060] In using the soluble tin electrode, it is possible to produce theSn(II) ions by dissolution consumed in the electrolytic depositionreaction so that the total amount of tin contained in the platingsolution is kept constant. As soon as the desired concentration ofSn(II) ions contained in the solution is achieved in the process ofanodic dissolution, the anodic dissolution reaction at the tin electrodecan be halted by interrupting the flow of current. After the current isno longer supplied to the soluble tin electrode, Sn(IV) ions may also bereduced at this electrode in causing them to react with the metallic tinof the electrode to form Sn(II) ions.

[0061] When using tin electrodes, the concentration of tin contained inthe plating solution, namely the concentration of Sn(II) ions, musthowever be analytically monitored with accuracy since otherwise, thedissolution of the tin electrodes may cause the concentration of tincontained in the plating solution to exceed the reference value. In thiscase, dissolution of metallic tin of the tin electrode is notautomatically limited which is the case when an inert auxiliary cathodeis exclusively used.

[0062] The tin plating solution may be contacted with the work indifferent ways: with conventional methods, the work is immersed into abath of the plating solution, which is filled in a container. In thiscase, the arrangement with auxiliary cathode and auxiliary anode islocated either in the same container in a free space or in a separatecontainer through which the plating solution flows. Fluid conduits inwhich the plating solution can be circulated between the treatmentcontainer and the regeneration container are provided for this purposebetween the treatment container and this other regeneration container.

[0063] Furthermore, the work can be treated in a so called horizontalplant with a coating chamber. In this horizontal plant, the work isconveyed in horizontal direction of transport through said chamber. Inthis case, the plating solution is delivered to the copper surfaces ofthe work by way of nozzles, e.g., spray nozzles, flow nozzles, jetnozzles or the like, while the work is conveyed through the chamber. Forthis purpose, the solution is kept in a reservoir from where it isdelivered to the nozzles by means of pumps. After the plating solutionhas contacted the copper surfaces, it is drained into collecting tanksfrom where it is returned to the reservoir via fluid conduits. In thiscase, the arrangement with auxiliary cathode and auxiliary anode isaccommodated either in the reservoir or in a separate regenerationcontainer.

[0064] Thus a method of depositing a layer of metal and a method ofregenerating a solution containing metal ions in a high oxidation state,especially a solution containing Sn(IV) ions, is described. Althoughspecific embodiments, including specific equipment, method steps, methodparameters, materials, solutions etc., have been described, variousmodifications to the disclosed embodiments will be apparent to thoseskilled in the art upon reading this disclosure. Therefore, it is to beunderstood that such embodiments are merely illustrative of and notrestrictive on the broad invention and that this invention is notlimited to the specific embodiments described, but only by the scope ofthe appended claims.

1. A method of depositing a metal layer, comprising the following methodsteps: a. Preparing a metal plating bath containing metal ions in a lowoxidation state; b. Depositing a metal layer from the metal plating bathonto a work piece; c. Bringing the metal plating bath in contact withthe metal serving for regeneration in order to reduce metal ion in ahigh oxidation state contained in the metal plating bath to metal ionsin a low oxidation state, wherein an electrolytic regeneration cellcomprised of at least one auxiliary cathode and of at lease oneauxiliary anode is provided and wherein the metal serving forregeneration is electrolytically deposited from the metal plating bathonto the at least one auxiliary cathode.
 2. The method of claim 1,wherein the method serves for depositing tin containing layers, whereinthe metal ions in the low oxidation state are Sn(II) ions and the metalions in the high oxidation state are Sn(IV) ions and wherein the metalis metallic tin.
 3. The method of claim 1, wherein the at least oneauxiliary cathode is made of copper or of a copper alloy.
 4. The methodof claim 1, wherein the at least one auxiliary cathode is made of aninert material.
 5. The method of claim 4, wherein the at least oneauxiliary cathode is made of platinized titanium.
 6. The method of claim1, wherein the metal is deposited in scales onto the at lease oneauxiliary cathode by adjusting the cathodic current density.
 7. Themethod of claim 1, wherein metal deposited onto the at least oneauxiliary cathode is mechanically removed and wherein, after removal,the metal is contacted with the metal plating bath in order to reducemetal ions in a high oxidation state contained in the metal plating bathto metal ions in a low oxidation state.
 8. The method of claim 1,wherein the at lease one auxiliary anode is separated from the spacesurrounding the at least one auxiliary cathode by a membrane.
 9. Themethod of claim 8, wherein the membrane is configured such that themetal ions may not permeate said membrane.
 10. The method of claim 8,wherein said membrane is an anion exchange membrane or a monoselectiveion exchange membrane.
 11. The method of claim 8, wherein an acid isprovided to the space surrounding the at least one auxiliary anode. 12.The method of claim 1, wherein at least one electrode containing themetal to be deposited is contacted with the metal plating bath andwherein the at least one electrode is polarized anodically relative toat least one further electrode so that the at least one electrodecontaining the metal to be deposited dissolves at least partially. 13.The method of claim 1, wherein the workpiece is conveyed in horizontaldirection through a coating chamber for deposition of the metal layer.14. A method of regenerating a solution containing metal ions in a highoxidation state, in which method the solution is brought into contactwith a metal serving for regeneration in order to reduce the metal ionsin a high oxidation state metal ions in a low oxidation state, whereinan electrolytic regeneration cell comprised of at least one auxiliarycathode and of at least one auxiliary anode is provide and wherein themetal serving for regeneration is electrolytically deposited from thesolution onto the at least one auxiliary cathode.
 15. The method ofclaim 14, wherein the method serves for regenerating a tin containingsolution, wherein the metal ions in the low oxidation state are Sn(II)ions and the metal ions in the high oxidation state are Sn(IV) ions andwherein the metal is metallic tin.
 16. The method of claim 9, wherein anacid is provided to the space surrounding the at least one auxiliaryanode.
 17. The method of claim 10, wherein an acid is provided to thespace surrounding the at least one auxiliary anode.
 18. A method ofdepositing a metal layer, comprising the following method steps: a.Preparing a metal plating bath containing metal ions in a low oxidationstate; b. Depositing a metal layer from the metal plating bath onto awork piece; c. Bringing the metal plating bath in contact with the metalserving for regeneration in order to reduce metal ion in a highoxidation state contained in the metal plating bath to metal ions in alow oxidation state; wherein an electrolytic regeneration cell comprisedof at least one auxiliary cathode and of at least one auxiliary anode isprovided and wherein the metal serving for regeneration iselectrolytically deposited from the metal plating bath onto the at leastone auxiliary cathode; wherein the method serves for depositing tincontaining layers, wherein the metal ions in the low oxidation state areSn(II) ions and the metal ions in the high oxidation state are Sn(IV)ions and wherein the metal is metallic tin; wherein the metal isdeposited in scales onto the at least one auxiliary cathode by adjustingthe cathodic current density; wherein the at least one auxiliary anodeis separated from the space surrounding the at least one auxiliarycathode by a membrane; and wherein the membrane is configured such thatthe metal ions may not permeate said membrane.
 19. The method of claim18, wherein said membrane is an anion exchange membrane or amonoselective ion exchange membrane; and wherein an acid is provided tothe space surrounding the at least one auxiliary anode.
 20. The methodof claim 19, wherein at least one electrode containing the metal to bedeposited is contacted with the metal plating bath and wherein the atleast one electrode is polarized anodically relative to at least onefurther electrode so that the at least one electrode containing themetal to be deposited dissolves at least partially.